Say the word Colorado and most people think of the mountains. With an average elevation of 6,800 feet, Colorado
is the highest state in the nation. In western Colorado, the continental divide forms an enormous Rocky
mountain chain with 59 peaks extending 14,000 feet or higher. The eastern portion of the State consists
of low-lying areas, resembling the flat plains of neighboring states like Kansas and Nebraska . In
between the mountains and the plains, Colorado's Front Range is aligned from north to south. This
region contains the largest cities and the majority of the population of Colorado. During the
winter months, the steep terrain offers abundant snowfall in the high country, but as summer
approaches, large temperature spreads and variations in topography can lead to an outburst of
severe weather, including the formation of tornadoes along the Front Range.

CLIMATOLOGY

Tornadoes are reported at least 9 months of the year, with June being the most active month. Tornadoes
are prevalent statewide, but most tornadic storms develop out over the eastern plains, east of the I-25
corridor. One wouldn't expect a tornado to rip through the heavily populated metropolitan area, but
it is more common than you might think. The population along the Front Range has grown drastically
over the years, which means tornadoes which used to strike in the open country, are now hitting
today's eastern suburbs. In fact, a tornado outbreak in Denver back in 1981, lead to the discovery
of a mesoscale phenomenon known as "The Denver Cyclone". After years of research, this feature was
formally named the Denver Convergence-Vorticity Zone (DCVZ). In this paper, I will aim to better
understand the DCVZ and its ability to spawn tornadoes along Colorado's Front Range.

FORMATION OF THE DENVER CYCLONE CONVERGENCE ZONE (DCVZ)

The American Meteorological Society defines the Denver Cyclone Convergence Zone as a mesoscale feature
of convergent winds, 50 to 100 km in length, usually oriented north-south, just east of the Denver
area. After years of analyzing the DCVZ, researchers are better able to understand the atmospheric
set-up that leads to the development of this convergence zone. The north-south wind component is
generated by a low-level southeasterly flow, combined with topographic forcing by a ridge of higher
terrain. That ridge is known as the Palmer Divide and extends eastward from the Front Range, just
south of Denver. The "Divide" juts out from the Rockies with an average elevation of 7600 feet. If
air is being forced from the SE, it is most likely a moist air mass moving up from the Gulf of
Mexico. Once this air is lifted over the divide, it converges with a northwesterly wind coming
off the foothills. This zone of converging winds can create small-scale cyclonic
vorticity. (Szoke et al. 1982). The diagram below illustrates "The Denver Cyclone" overlaid
on a topographic map. As you can see, the SE wind component flows up over the Palmer
Ridge, otherwise known as the Palmer Divide. As the air flow meets up with the wind
coming down off the Front Range foothills, a zone of convergence develops, as indicated
by the blue dotted line.

TORNADOGENESIS

Climate data from the 1980s indicates that when a well-formed DCVZ is present in June, there is a 70% chance
of a tornado forming somewhere in or near the zone. (Edward J. Szoke, June 1997 FSL Forum). Storms created
by the DCVZ are a major concern for Denver, especially during the spring and early summer. Deadly cloud
to ground lightning strikes, heavy rain, possible flooding, severe hail and tornadoes are all possible
when this feature is present. One particular case study dates back to the afternoon of
June 3, 1981. On this unusually humid day, two F2 tornadoes moved over portions of the
populated metropolitan area. Details on this particular event, including 10 other nonsupercell
tornadoes in the vicinity of the Denver International Airport all appear to have a connection
to the DCVZ. (Szoke, E. J. in 1982).

In addition to the studies above, eight separate DCVZ events were analyzed by the Mobile Mesonet during the
summer of 1998. Based on the observations, the boundary varies in horizontal extent at measurable
scales from ~100 m to a region several km across. Observations also yielded numerous cyclonic eddies
traveling along the boundary, ranging in size from 200 m to 4 km in diameter. Of the eight events, a
total of 11 vortices were identified. (Albert E. Pietrycha and Erik N. Rasmussen).

CONCLUSION

Continued research is our only means of understanding the DCVZ. While the synoptic scale features
associated with this phenomenon are well documented in several of the studies above, translating that
into an accurate forecast of tornadogenesis will always be a challenge. Most importantly, it is crucial
to know the topography of your forecast area, especially in a state as variable as Colorado. Whether
it is upslope flow resulting in substantial snow amounts, or tornadogenesis in relation to
"The Denver Cyclone," understanding the region will play a big role
in delivering accurate forecasts.

Complex radar networks will help to provide better coverage of the DCVZ boundary and vertical vorticity, but
proximity to the radar will always be a limiting factor.

REFERENCES

AMS Glossary of Meteorology

"A Subsynoptic Analysis of the Denver Tornadoes of 3 June 1981," by E.J. Szoke, National Center
for Atmospheric Research, Boulder CO